It is well known that in the manufacture of rubber products crude or raw rubber is compounded with various ingredients, among which are sulfur and vulcanization accelerators. The primary function of an accelerator is to increase the rate of the vulcanization reaction. In addition, the accelerator must be of such a nature that it provides sufficient time to enable admixture with the rubber at an elevated temperature before vulcanization commences. This is commonly referred to as scorch time. Secondary effects are almost of equal importance and include improved tensile strength properties, set, hysteresis, and ageing, among others. The compounder is likely to be concerned with scorch behavior, cure rate, the extent and time of vulcanization, compatibility, and the like, when using any one or a combination of accelerators in a vulcanization process.
Since some accelerators have too slow a cure rate when used alone, they may require the use of an additive to speed up or activate the cure rate. This can be achieved by adding small amounts of another accelerator, commonly referred to as an activator. The accelerators which most commonly require activation are the thiazoles, including benzothiazole-2-sulfenamides, while those most commonly used as activators are the thiuram sulfides, dithiocarbamates, and guanidines. The primary accelerators, including the benzothiazole-2-sulfenamides, are usually employed in a concentration of about one or two parts per hundred of rubber, while the activators are employed in a concentration of about 0.1 to 0.25 parts per hundred of rubber. This has led to the characterization of the activators as secondary accelerators.
Among the primary accelerators, sulfenamides, particularly benzothiazole-2-sulfenamides, are widely used in the vulcanization of a wide range of rubber compositions. They are frequently used in combination with thiuram sulfides as secondary accelerators. The latter are powerful activators and produce a strong activating effect even at very low concentration. Consequently, they tend to be scorchy, that is, they cause premature vulcanization. The effect of both factors is to use the activators in small amounts. A ratio of 5-10 to 1 of thiazole sulfenamide to activator is common. Since they are such powerful activators, small weighing errors as well as incomplete dispersion in rubber can, and sometimes does, lead to serious consequences, causing under-cured parts or premature vulcanization (scorched stock).
From time to time in the past, interest has been generated on the part of both compounders and suppliers in pre-blended mixtures of primary accelerators and activators in the proper, commonly used weight ratios. They have generally not been acceptable for use because such blends have tended to be unstable on storage, losing a very significant amount of accelerator activity in a relatively short time, particularly at moderately elevated temperatures. The loss of activity if manifested most significantly in rate of cure, that is, the accelerator blends require extended vulcanization times to develop optimum physical properties. Thus, the desired effect of the activator, i.e., to give shorter cure time, is lost. Often times the extent of cure and scorch time are affected to a similar degree. In this case, not only is the cure rate lower, but the stock becomes more scorchy, or prone to premature vulcanization. The state of cure is often lower as well. Thus, while many primary accelerators, particularly benzothiazole-2-sulfenamides, are not highly storage stable, their blends with thiurams are much less stable.
Interest in such blends persists, nonetheless, and the present invention represents the culmination of investigations directed to finding means for stabilizing accelerator blends.
Accordingly, it is a primary object of the present invention to provide stable blends of primary accelerators, particularly benzothiazole-2-sulfenamides, and thiuram sulfides.
Another object of this invention is to provide a method for stabilizing blends of primary accelerators, particularly benzothiazole-2-sulfenamides, and thiuram sulfides.
Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and obtained by means of the compositions, processes, and improvements particularly pointed out in the appended claims.